Synthetic lethality describes a genetic interaction in which simultaneous mutations in two genes lead to synergistic cell death compared to individual mutations in the same genes (Kaelin, 2005; Yang et al., 2008a). The concept of synthetic lethality was originally used to study the buffering capacity of cells and organisms upon genetic variations, through which many gene-gene interactions have been discovered in multiple organisms, including bacteria, yeasts, and nematodes (Dixon et al., 2009; Malumbres, 2003). Soon after, it was recognized that this concept can be used as a framework for discovering anti-cancer drug leads with high therapeutic indices (Kaelin, 2005; Hartwell, 1997): one can search for small molecules that are only lethal in the presence of a specific oncogenic mutation.
Oncogenic RAS proteins have been targeted using this synthetic lethal screening approach, due to the widespread importance of mutant RAS proteins in the genesis and maintenance of human cancers (Malumbres et al., 2003), as well as the challenge of targeting oncogenic RAS proteins directly (Downward, 2003). Several synthetic lethal screens using RNA-interference-based (RNAi) libraries reported genes with synthetic lethal relationships with KRAS, such as PLK1 (Luo et al., 2009), TBK1 (Barbie et al., 2009), STK33 (Scholl et al., 2009), and GATA2 (Kumar et al., 2012). Some of these results may require further verification, because some follow-up studies did not support the originally postulated roles (Babij et al., 2011; Luo et al., 2012). The mechanism of synthetic lethality was attributed to increased dependence on mitotic function, NF-κB signaling, S6 kinase activity, and the GATA2 transcriptional network, respectively. The specific death-initiating mechanisms were different in these cases; however, cancer cells with oncogenic RAS mutations invariably died via apoptosis upon treatment with these RNAi reagents.
A different approach to targeting oncogenic RAS uses synthetic lethal screening with small molecules. Several RAS-synthetic-lethal (RSL) compounds were identified using this strategy (Yang et al., 2009; Yagoda et al., 2007; Weiwer et al., 2012; Shaw et al., 2011; Ji et al., 2009). The lethality of these RSL compounds, such as erastin and RSL3, was significantly enhanced upon activation of RAS-RAF-MEK signaling. In contrast to the results of RNAi screens, the small molecule approach yielded compounds that induced a distinct form of oxidative, non-apoptotic cell death. This mode of cell death was distinct from necrosis, and is a regulated form of oxidative cell death termed ferroptosis due to its unique morphology, inhibitor sensitivity and strict dependency on iron (Dixon et al., 2012). Thus, ferroptosis may be an efficient means of inducing synthetic lethality with small molecules in tumor cells harboring oncogenic RAS proteins. Defining the molecular pathways governing ferroptosis could aid in targeting RAS mutant tumors.
To define the core effectors of ferroptosis, erastin and RSL3 were further investigated, because both of these RSL compounds induce ferroptotic cell death via different triggering mechanisms. Erastin binds to VDAC2/3 (Yagoda et al., 2007), and inhibits system xc− (Dixon et al., 2012) to induce ferroptotic cell death. In contrast, RSL3 is not dependent on these proteins (Yang et al., 2008a), and its target has not been reported. Metabolomic profiling was used to evaluate comprehensively changes in metabolism upon erastin treatment, and it was found that a common lipoxygenase-mediated pathway executing ferroptotic cell death in response to RSL compounds.
RAS genes are among the most commonly mutated in human cancers, but their protein products have remained intractable to therapeutic agents. Thus, there is a need for, inter alia, anti-cancer drugs with high therapeutic indices that selectively target tumor cells, such as those harboring oncogenic RAS mutations. The present invention is directed to meeting these and other needs.